This invention relates to an improved flexible tape transducer.
The electret tape transducer described in U.S. Pat. No. 4,023,155 has the advantages of being flat, flexible and of providing unidirectional object detection capability so that it operates both as an active and as a passive transducer. This tape transducer, however, has a disadvantage in that proper electrical shielding requires that a conducting strip be located on the side of the inner strip opposite from the moving conducting strip and be electrically connected to the latter along the marginal edges. For purposes of discussion, this transducer design can be considered as an upper portion and a lower portion with the inner conducting strip on the boundary between them. The upper portion provides the transduction function while the lower portion is inactive. However, since the basic transducer when used to detect vibrations is a charge generator and since the inactive capacitance of the lower portion is in parallel with that of the active portion, the output voltage of the transducer is reduced by the presence of the shielding strip. For example, if the active and inactive capacitances are equal, the output voltage of the transducer is reduced by 6 dB.
The inactive capacitance may be reduced to a negligible amount by sufficiently increasing the spacing between the shielding strip and the inner strip and by using as a spacer a low dielectric constant material such as plastic foam. The difficulty with this approach is that the thickness of the tape transducer is substantially increased and the flexibility and rollability of the device are substantially reduced. Both of these results are undesirable and in some cases unacceptable. Another disadvantage of this design is the requirement that the center strip have high inertia relative to that of the moving strip. This requirement is achieved by using a thick, heavy metal tape which increases the cost of the transducer.
This invention is directed to an improved tape transducer that overcomes these problems.